1. Field of the Invention
The present invention relates to a testing head having vertical probes and used to test a plurality of semiconductor-integrated electronic devices incorporating so called contact pads.
2. Description of the Related Art
As is well known, a testing head is basically a device suitable to electrically interconnect a plurality of contact pads of a semiconductor-integrated electronic device and corresponding channels of a testing machine arranged to perform the tests.
Integrated electronic devices are factory tested in order to spot and reject any circuits that show out to be already defective during the manufacturing phase. The testing heads are normally employed to electrically test the semiconductor-integrated electronic devices xe2x80x9con waferxe2x80x9d, before cutting and mounting them in a chip package.
A testing head having vertical probes comprises at least a pair of parallel plate-like holders placed at a given distance apart to leave an air gap therebetween, and a plurality of specially provided movable contact elements.
Each plate holder, referred to as a die in the art and throughout this specification, is formed with a plurality of through-going guide holes, each hole in one of the dies corresponding to a hole in the other die and guiding a respective contact element, or contact probe as the element will be called through this specification and the appended claims, for sliding movement therein. The contact probes are usually cut from wire stock of some special alloy having good electrical and mechanical properties.
A good electrical connection of the testing head contact probes to the contact pads of an integrated electronic device to be tested is achieved by urging each contact probe onto the respective contact pad. This results in the movable contact probes becoming flexed in the air gap between the two dies.
Testing heads of this type are commonly known as xe2x80x9cvertical probesxe2x80x9d.
Briefly, known testing heads have an air space where the probes are allowed to flex, such a flexion action being eventually assisted by suitable design of the probes or their dies, as shown schematically in FIG. 1.
As shown in FIG. 1, a testing head 1 comprises at least an upper die 2 and a lower die 3, both dies being formed with through-going upper guide hole 4 and lower guide hole 5, respectively, in which at least one contact probe 6 slides.
The contact probe 6 has a contact end or tip 7. In particular, the contact tip 7 is caused to abut against a contact pad 8 of an integrated electronic device to be tested, thereby establishing an electrical contact between said device and a testing apparatus (not shown) that has said testing head as end element.
The upper and lower dies 2 and 3 are suitably separated by an air space 9 in which the contact probes 6 are allowed to deform or flex in normal operation of the testing head, i.e. upon the testing head coming in contact with the integrated electronic device to be tested. The upper and lower guide holes 4 and 5 are both sized to guide the contact probe 6.
FIG. 1 schematically shows a testing head 1, which mounts loose-fitting probes and is associated with a micro-contact strip or space transformer shown schematically at 10.
In this case, each contact probe 6 has another contact tip toward a plurality of contact pads 11 of the space transformer 10. The electric connection of the probes to the space transformer 10 is assured same as the connection to the integrated electronic device to be tested, i.e. by urging the probes 6 onto the contact pads 11 of the space transformer 10.
A major advantage of a testing head 1 with loose-mounted contact probes is that one or more faulty probes 6 in the set of probes, or the whole set, can be replaced more conveniently than in testing heads that have fixed probes.
In this case, however, the upper and lower dies 2 and 3 should be designed to ensure that the contact probes 6 will be held in place even when no integrated electronic device is abutting their contact tips 7 for testing, or when a probe set is removed for replacement purpose.
The deformed pattern of the probes and the force needed to produce the deflection depend on several factors, namely:
the distance between the upper and lower dies;
the physical characteristics of the alloy from which the probes are formed; and
the amount of offset between the guide holes in the upper die and the corresponding guide holes in the lower die, as well as the distance between such holes.
It should be noted that, for the testing head to perform properly, the probes should be allowed a suitable degree of free axial movement within the guide holes. In this way, the probes can also be taken out and replaced individually in the event of a single probe breaking, with no need to replace the whole testing head.
All these features are, therefore, to be taken into due account in the manufacture of a testing head, given that a good electric connection between the probes and the device to be tested is mandatory.
Also known is to use contact probes having a pre-deformed shape even when the testing head 1 is not contacting the device to be tested, as in the probes 6b, 6c and 6d shown in FIG. 2A. The pre-deformed shape effectively helps the probe to correctly flex during its operation, i.e. upon contacting the integrated electronic device to be tested.
Conventional testing heads inherently place limits on the distance to be lowered between two adjacent probes 6, while the technological development and the chip miniaturization continuously press to reduce the distance between centers of two contact pads 8 of an integrated electronic device to be tested, this distance being known as the pitch distance of the pads.
Thus, a minimum pitch, in the sense given above, will be dependent on the layout and the dimensions of the probes 6, according to the following relation:
Pitchmin=E+2Amin+Wmin
where Amin=(Fxe2x88x92E)/2 and where, as shown in FIG. 2B, which is a sectional view through part of a testing head 1 according to the prior art:
Pitchmin is the minimum pitch or distance between centers of two adjacent contact pads 8 of the integrated electronic device to be tested;
E is the dimension of the cross-section of the probe 6. For example, in probes having a circular cross-sectional shape, the dimension used for computing the minimum pitch would be the cross-section diameter value of the probe 6, where the probe has a square cross-sectional shape, while in probes having a rectangular cross-sectional shape, the dimension used for computing the minimum pitch would be the minor side or the major side of the rectangular cross-section of the probe 6, depending on the chosen arrangement for positioning the contact probes;
In particular, FIGS. 2C, 2D, 2E and 2F are top plan views of a testing head portion comprising contact probes 6 having a circular cross-sectional shape (FIG. 2C), a square cross-sectional shape (FIG. 2D) and a rectangular cross-sectional shape (FIGS. 2E and 2F, in mirrored configurations) respectively.
Amin is the minimum distance between a probe 6 and its guide holes 4e 5 that allows the probe to slide freely in the guide holes 4, 5 during normal operation of the testing head;
Wmin is the minimum wall thickness allowable between one guide hole 4, 5 and the following, in order to guarantee the testing head 1 to be an adequately strong structure; and
F is the dimension of the cross section of a guide hole 4.
Current vertical technologies, usually with circular cross-sectional shaped probes, achieve a reduction of the pitch value by reducing the dimensions, and especially reducing the minimum dimension E (being the minimum diameter for probes having a circular cross section) of the probes 6. The other factors in the above relation are set practically by technological limitations to the manufacture of the testing head.
An embodiment of this invention provides testing heads for microstructures, which comprise probes designed to deform upon coming in touch with contact pads in order to establish a good electric connection to an integrated electronic device to be tested, and adapted to allow a substantial reduction in the distance between contact tips and thus a reduction in the pitch distance between contact pads of integrated electronic devices to be tested.
One of the principles on which an embodiment of the present invention stands is to provide a testing head with a plurality of vertical probes having at least a rigid end portion extending laterally with respect to the contact probe body.
Presented is a testing head having vertical probes and comprising a first and a second plate-like holder provided with respective guide holes a contact probe adapted to be received in the guide holes and having a contact tip adapted to establish mechanical and electrical contact to a corresponding contact pad of an integrated electronic device to be tested, the contact probe being deformed in a deflection region located between the plate-like holders as the contact tip abuts onto the contact pad wherein the contact probe further comprises a rigid arm extending laterally from a body of the contact probe and terminating in the contact tip, the rigid arm being adapted to offset the contact point of the contact probe with the corresponding contact pad with respect to a longitudinal axis of the contact probe.
The features and advantages of the testing head according to this invention will be apparent from the following description of embodiments thereof, given by way of non-limitative examples with reference to the accompanying drawings.